1. Field of the Invention
The present invention relates generally to electrostatic filtering systems, and in particular, to electrostatic filtering systems for collecting and removing fine, particulate matter from hydraulic fluids, and other insulating and/or dielectric fluids.
2. Description of Related Art
Many mechanical and electrostatic filters exist for removing contaminants from oils, hydraulic fluids, insulating and other insulating and/or dielectric fluids. These contaminants include ions, moisture, molecular impurities and particulate matter within such fluids. In purely mechanical filtration systems, a fluid flow passes through one or more filter materials, whereby these filters trap and remove contaminants from the fluid flow. However, since very fine particulate matter, such as particulate matter having dimensions of less than about 3 to 5 microns, passes directly through the larger size pores of the mechanical filter medium, these types of systems are not efficient in trapping and removing such fine particulate matter. Another problem associated with mechanical filtration systems is that the small particle size of the contaminants requires large bulk and volume filters to avoid excessive pressure drop caused by the smaller openings in the filter media.
As an alternative to mechanical filtration systems, electrostatic filters have been implemented to remove fine particles by passing the contaminated fluid over or through a plurality of perforated electrodes, which are alternately charged positive and negative. In some of the known electrostatic filters, porous filter media is placed between the electrodes for trapping the particulates and extend across the entire internal cross-sectional area of the filter. Filtration in these types of systems is achieved by the generation of an electric field between adjacent electrodes to charge the filter media and attract the particulate matter to such charged filter media. Alternatively, the particulate matter is charged positive or negative so that oppositely charged particles attract to each other and floc into clumps (i.e., flocculate). Filter media then mechanically filters out these clumps or flocs from the fluid flow. When enough clumps form to effectively block the filter or produce an undesirable pressure drop, the filter media must be replaced.
Electrostatic filters of this type are shown in U.S. Pat. No. 4,594,138 issued Jun. 10, 1986 to Donald E. Thompson, U.S. Pat. No. 5,332,485 issued Jul. 26, 1994 to Donald E. Thompson, U.S. Pat. No. 5,630,926 issued May 20, 1997 to Donald E. Thompson, U.S. Pat. No. 5,785,834 issued Jul. 28, 1998 to Donald E. Thompson, U.S. Pat. No. 6,129,829, issued Oct. 10, 2000 to Donald E. Thompson, U.S. Pat. No. 6,284,118, issued Sep. 4, 2001 to Donald E. Thompson, and U.S. Pat. No. 6,576,107, issued Jun. 10, 2003 to Donald E. Thompson.
These patents generally disclose contaminated fluid flowing axially through a filter, whereby layers of filter media separate perforated electrodes in a single filter element. The perforated electrodes are alternately oppositely charged, with the filtration process taking place by flowing the contaminated fluid upwardly through perforations in the electrodes and the filter media between the plates in this single element. These patents also disclose electrostatic charging of the particles within the fluid as a result of direct electrical connection of a power supply to the perforated electrodes within the filter element to generate electric fields therein that are imposed upon the fluid flow and/or the filter media. The fluid flows through these electric field inside the filter element, before or during flow of the fluid through the adsorbent material, whereby charged particles and large sized particles (i.e., those having diameters up to about 254 microns) are able to flow directly through the large openings in the filter media and out the filter. Those particles exiting the filter agglomerate with oppositely charged particles, whereby these agglomerates are then mechanically filtered out of the fluid flow in another element having filter media.
Accordingly, while perforated electrostatic filters may be effective to a certain extent, they have certain drawbacks and inefficiencies, as discussed above. Additionally, perforated electrostatic filters have limited amounts of surface area for filtration, and as such, if water or other contaminants reach a level sufficient to permit short circuits between the perforated electrodes, or plug the filter media, the filter is rendered less effective or even useless.
These types of filters also do not adequately solve the hydraulic problems attributable to particulate contaminants having dimensions of less than about 3 to 5 microns in diameter. Many modern oils, hydraulic fluids, and other insulating and/or dielectric fluids generate static electricity as a result of the friction of the fluid flowing through the filter system. Accumulation of this static electricity leads to spark discharge, which deleteriously causes the contamination problems of both very fine charged particles and oxidation of the fluid. Current electrostatic filters are not effective in trapping and removing such fine particulate matter from the fluid flow.
Therefore, a need continues to exist to have improved electrostatic filter systems for removing very fine particulate contaminants from hydraulic fluid and other insulating fluids, and in particular, to systems that are effective in eliminating the hydraulic problems associated with sub micron contaminant particles. Accordingly, a continuing effort has been directed to the development of improved electrostatic filters.